Multiple cell photovoltaic semiconductor devices are well known. Single crystal devices of this sort were described by E. D. Jackson in the Transactions of the Conference on the Use of Solar Energy in Tucson, Ariz. in 1955. Typically the multiple cell devices use two or more solar cells electrically and optically connected in series. Solar radiation or other light energy enters the first cell. There, a spectral portion of the light is absorbed and electrical power is generated in response to the absorption. The unabsorbed light that passes through the first cell reaches the next cell in the device where another portion of the light spectrum is absorbed. The process is repeated through each of the cells in the device. Each of the cells must have a different absorption characteristic, i.e. optical band gap, in order to achieve the goal of absorption of different spectral components in each of the cells. This goal can be accomplished by using different semiconductor materials in different cells, by adjusting the bandgaps in different cells by adding a bandgap adjusting material to certain cells, or by other means.
The known multiple cell structures have two or more electrical terminals. In the two terminal device, the current generated by the light absorption flows through each of the series-connected cells and is therefore constrained to be the same. To maximize device efficiency, each cell must generate the same current (the photogenerated voltages add), a goal that is achieved by properly selecting the relative optical bandgaps of the different cells and/or the thicknesses of the cells thereby affecting the relatively quantities of absorbed and transmitted light in each cell. Two terminal devices require a non-blocking junction be disposed between each of the cells in the device. By contrast, devices having more than two terminals, have blocking (i.e. non-ohmic or electrically insulating) junctions between the cells. These devices, which are not further dealt with here, have a pair of electrical terminals connected to each cell in the device.
Examples of multiple cell devices of the type just described and employing thin films are disclosed in the following U.S. patents. U.S. Pat. Nos. 4,272,641 and 4,316,049 to Hanak disclose amorphous silicon devices employing two cells separated by a cermet. The cermets are thin sputtered layers, e.g., of metal silicides, that are optically transmissive and electrically conducting. U.S. Pat. No. 4,377,723 to Dalal discloses two cell devices, each cell of which comprises three layers of amorphous silicon, the then outer layers being oppositely doped and spanning a relatively thick non-doped layer. The cells are disposed either directly in contact so that a tunnel junction is formed between them or the cells are separated by a solid, electrically conductive layer transmissive to light that is not absorbed in the cell through which light first passes. The intervening solid layer is preferably amorphous silicon. The bandgap of the lower cell, i.e. the second one the incident light enters, is narrowed by the inclusion of germanium in the amorphous silicon. U.S. Pat. No. 4,479,028 to Sato et al. discloses a three cell device formed from amorphous and microcrystalline silicon. Different spectral absorption properties for the cells are achieved by varying the thicknesses of the layers and cells, adding germanium to the lowest cell and using microcrystalline silicon or adding carbon in the shallowest cell. U.S. Pat. No. 4,536,607 to Wiesmann discloses a two cell device having one cell formed of three layers of an amorphous material and a second cell formed of a two layer polycrystalline semiconductor heterojunction. The two cells are separated by a layer of an electrically conducting, transparent oxide, a cermet, or a very thin layer of a metal. The patent to Wiesmann proposes sequential deposition of the layers in the structure including the layer separating the cells.
Multiple cell, thin film devices show the greatest promise for achieving high efficiency photovoltaic performance. Of those devices, the most promising are those containing different materials (e.g. a polycrystalline II-VI material in one cell and an amorphous or ternary polycrystalline material in another cell), because the optical bandgap selection opportunities are greater when a wider variety of materials is available for use. The limitation on selection of various materials for use in multiple cell devices is the compatibility or lack of compatibility of preparation techniques for the different materials chosen. That is, many combinations of cells, each made of different materials have been proposed, but no one has been able to construct many of these devices because the condition for preparing one of the necessary materials in the device may destroy the usefulness of another. For example, thin film multiple cell structures have been traditionally manufactured by sequentially depositing layer upon layer. But that sequential deposition process cannot be used when the depostion of a subsequent layer spoils previously deposited layers because of the temperature, ambient, or reactants used. This limitation has restricted the actual construction of many preferred, high efficiency multiple cell devices that have been proposed, including some of the devices described by Wiesmann in U.S. Pat. No. 4,536,607.